1. Field of the Invention
This invention related to a reversed Galilean optical system and, in particular, to a compact reversed Galilean finder optical system comprising two lens units of an objective and an eyepiece.
2. Description of the Related Art
A reversed Galilean optical system consists of an objective unit with a negative refracting power and an eyepiece unit with a positive refracting power. This optical system, although simple in arrangement, enables an image with a wide field angle to be observed at a time and thus has been used as a finder for cameras for a long time. Designating by f.sub.1 and f.sub.2 the focal lengths of the objective and eyepiece ulnas of the reversed Galilean optical system, respectively, the ratio between them, -f.sub.1 /f.sub.2, indicates the magnification. The distance between the paraxial principal points of the optical system is determined by the sum of the focal lengths of its constituent lenses, f.sub.1 +f.sub.2, which is paraxially the overall length of the optical system.
Based on the relationships stated above, the arrangement and technique for compact design of a reversed Galilean finder are set forth, for example, in Japanese Utility Model Preliminary Publication No. Sho 55-55829. This device is intended to downsize the reversed Galilean finder by reducing the focal lengths of its constituent lenses. Also, arrangements and techniques for improving the performance of the reversed Galilean finder by using aspherical surfaces in the objective units are mentioned in Japanese Patent Publication Nos. Sho 61-35533, Sho 61-40087, and Hei 3-20732, and Japanese Patent Preliminary Publication Nos. Sho 50-87027, Sho 52-62023, Sho 55-93116, and Sho 63-71822.
In the reversed Galilean optical system, where the magnification is determined as a precondition, the design of the optical system which decreases in overall length requires the use of the combination of lenses such that the focal length ratio between the objective and eyepiece units is constant and their focal lengths are reduced. If, however, the overall length of the optical system is diminished by such a combination, there is the problem of increasing the degree of correction for aberration in each lens. Further, even though the aspherical surface is used in the objective unit, it is impossible to completely correct for aberration produced in the eyepiece unit with accompaniment of the reduction of the overall length. Thus, there is a limit to compact design with full performance as well.
In the reversed Galilean finder, as mentioned above, the design of the finder whose overall length is reduced is very disadvantageous in view of correction for aberration, and the lens arrangement of some degree of length is essential for holding full performance. Moreover, the effective diameter required for the objective unit increases with increasing length of the lens arrangement. Hence, there is the problem that the finder, and therefore the entire camera, becomes bulky.